Among titanium or titanium alloy base materials are pure titanium (e.g. JIS Class 1 and 2) and high-strength titanium alloys exemplified by α-β alloys, 6-4 alloys (e.g. JIS Class 60), β alloys, and 15-3-3-3 alloys. Among titanium or titanium alloy base materials used for medical metallic devices are 6-4 alloys and ELI (Extra Low Interstitial Elements) materials that are 6-4 alloys containing oxygen, nitrogen, hydrogen, and iron in particularly reduced amounts. The 6-4 alloys and ELI materials possess a relatively high strength and consistently maintain their high strength even at a high temperature, but are difficult to machine, easily wear, and are liable to develop seizure or galling.
A medical device made of a titanium or titanium alloy base material is manufactured by cutting or otherwise machining a block material, which is produced by, for example, rolling, into a given shape.
Cutting is implemented using an end mill. The surfaces of a medical device need to be so made that they inhibit adhesion of germs to the medical device and, when in contact with fluent blood, inhibit adhesion of platelets to the medical device and, hence, formation of blood clots. Cutting a titanium or titanium alloy base material surface with, for example, an end mill size-reduces crystal grains in the surface and leaves traces of cutting, thereby posing great problems in obtaining a surface inhibiting adhesion of germs as required of medical devices.
To address the above problems, the machining is followed by, for example, buffing, chemical etching, or blast polishing.
However, these processes in turn present their own problems: buffing is not applicable to a base material having a relatively complicated configuration; chemical etching exposes crystal grains of a base material; in blast polishing, pieces of a blasting material drive into a base material and remain in the base material surface. Polishing requires extended labor.
Until now, there is no known method of manufacturing a medical device using an electron beam irradiation process to reduce platelet adhesion of surfaces.
Japanese Patent Laid-open No. JP 2003-111778 and “Development of Dental Metal Surface Polishing Method using Electron Beam,” a PhD dissertation by Junko TOKUNAGA, Graduate School of Dentistry, Osaka University, March 2008, disclose a known method involving subjecting a metallic base material of pure titanium, a metal used in dentistry, to electron beam irradiation for improved surface flatness, enhanced glossiness, and increased corrosion resistance.
However, when a material other than pure titanium, chiefly a 6-4 titanium material or the like, is subjected to electron beam irradiation for improved surface flatness, enhanced glossiness, and increased corrosion resistance in the same manner as used for pure titanium, impurities contained in the titanium material may develop small pits (hereinafter referred to as craters) in the outermost surface of the material as the outermost surface is caused to boil and vaporize by the electron beam irradiation, resulting in defects in the surface.